The invention relates to a method and apparatus for improving the resolution of digital signals, in particular luminance and/or chrominance TV signals, the scanning values of which are transformed in blocks, for example by discrete cosine transformation, in such a way that one coefficient representing a uniform component and several coefficients representing alternate components are created which are in the course of calculation limited to a smaller number of places, i.e., digits or bits, rounded off and inversely transformed after transmission and/or storage. A uniform component is also known in the art as DC-coefficient and an alternate component is known as AC-coefficient.
In order to reduce the quantity of data during the digital transmission of TV signals, it is known to initially transform the TV signals in blocks from an original range to a selected frequency range, then to transmit only those spectral coefficients obtained by means of the transformation which are essential and to zero out the nonessential ones, and finally to inversely transform the transformed values back into the original range. Transformation takes place by a multiplication of the signal values with a transformation function, the number of places, of bits, increasing because of the transformation. In order to reduce the number of places to a set value for a sufficient resolution of the gradations of the luminance and/or chrominance signal values, for example 8 bits plus sign, a two-dimensional integer division of the coefficient blocks by eight, and thus a rounding off of the transformation values, is performed in connection with, for example, a two-dimensional transformation of signal blocks, which may each have a size of, for example, 8.times.8 pixels, i.e., each block represents a rectangular picture area composed of 8 rows and 8 columns of pixels.
Such a technique is described, for example, by G. Bostelmann, et al., in an article entitled "Codierung von Videosignalen [Coding of Video Signals]" in Elektrisches Nachrichtenwesen, Vol. 59, No. 3, 1985, pages 286-294.
It has been shown that with small alternate components of the transformed coefficients, i.e., with smooth patterns of the original picture, coarser patterns appear after inverse transformation. The reason for this is that because of the rounding off of the transformed values, information is lost, and thus, is lacking after inverse transformation, and becomes visible as interference because of a lack of masking by signal components with higher frequencies.